Oscillators are widely used as stable frequency sources in many diverse electronic applications. For example, in communication systems, oscillators are often used to provide a stable reference signal for converting information signals to the proper frequency band. In conventional multi-channel communication systems, circuit arrangements employing multiple oscillators interconnected with various circuit elements are used to provide a selectable frequency source. This approach is generally used for high speed switching applications. For less critical applications, a more economical approach entails the use of a tunable oscillator circuit comprising a voltage controlled oscillator ("VCO") which can be phased locked to the harmonics of a reference signal. The performance of these systems, however, is highly dependent on the frequency stability of the VCO. Phase noise in the VCO output severely limits the accuracy of the entire system.
To reduce the effects of phase noise, VCO circuits of the past have been augmented with negative feedback. U.S. Pat. No. 4,336,505 by Meyer is an example of such an approach. A VCO with a time-delay feedback loop provides a noise degenerated frequency source. Because of the periodic response of the time-delay feedback loop, stable oscillation can be achieved at many frequencies across the band of interest. FIG. 1A illustrates in qualitative terms the frequency spectrum of a VCO at 10 MHz. employing a time-delay feedback loop. For many of today's applications, however, these noise sideband levels are simply too large. For example, consider an X-band radar designed to detect slow moving objects such as trucks, tanks and helicopters. In this application, the noise sidebands must be reduced by at least 40 dB to detect a 4 kilometer/hour target.
Crystal oscillator technology offers frequency stability amenable to such applications. A crystal oscillator comprises a piezo-electric crystal, usually quartz, sandwiched between two electrodes. When properly excited, an electromechanical reaction occurs causing the crystal to vibrate at the excitation frequency. Extraordinary high values of Q are obtained which, in conjunction with the fact that the characteristics of quartz are extremely stable with respect to time and temperature, account for the exceptional frequency stability.
A growing concern has recently been recognized concerning the effects vibration on frequency stability of crystal oscillators for radar systems, communication links, and other sensors. Random and sinusoidal vibration has been generally experienced when these systems are operated in mobile environments. This is due to the vibrational effects on the quartz crystal resonator in the oscillator. When the crystal undergoes vibration, frequency modulated sidebands are produced. These sidebands appear in the form of phase noise which severely degrades the spectral purity of oscillation, and significantly limits the performance of the systems in which they are used. In fact, the sensitivity of crystal oscillators to vibration is so severe that in some systems, vibrations of cooling fans in fixed ground stations, or footsteps in corridor traffic, have been shown to degrade their performance.
The basic problem is illustrated in FIG. 1B which shows the degradation of a 10 MHz. STAMO of an X-band radar at rest and when operating in a 1 g vibration environment. As shown in FIG. 1B, a 4 kilometer/hour target can be detected with a 10 MHz crystal oscillator at rest. However, when the crystal oscillator is introduced into a 1 g vibration environment, the improvement in phase noise achieved with crystal oscillators over conventional VCO circuits with feedback is lost. In this example, the noise sidebands must be reduced by at least 53 dB to detect the target. The situation is aggravated in millimeter wave systems.
Several techniques have been employed in the past in an attempt to desensitize the oscillator frequency from the effects of vibration. Designs involving shaping of the resonators and their electrodes, selection of materials, orientation of the surfaces of the crystal relative to the crystalline axes, etc. have been employed. Circuits utilizing multiple resonators placed back to back to cancel the effect of acceleration have also been proposed. However, these approaches are limited because of the significant increase in cost of the resonator or the reduction of the resonator quality factor Q.
Another approach involves the mechanical isolation of the resonator from the vibration environment. Structures have been designed within a cabinet that are mechanically isolated from the vibrations being transmitted through the equipment. These vibration isolators essentially act as low pass filters, providing good isolation at high vibration frequencies, and none at low frequencies. If poorly designed, vibration isolators can actually amplify the vibration source at frequencies where mechanical resonances exist.
There are numerous other problems associated with the mechanical isolation of resonators. By way of example, mechanical vibration isolators require space, which is a premium in many systems, particularly in airborne systems. In addition, the measures which are introduced to improve vibration isolation generally are deleterious under shock conditions, e.g., they can lead to large physical displacements and instability. Finally, the electrical connections act as stiffeners, and degrade the isolation of the mechanical isolators.
Other approaches to control the effects of shock and vibration on crystal oscillators have also been attempted. In one such approach, the output of an accelerometer is fed back into the crystal oscillator to effectively cancel the frequency modulation. Since this arrangement employs an open loop circuit, stability is compromised. In addition, it is practically impossible to align the accelerometer so that it experiences the same vibrational effects as the crystal oscillator. Further, the acceleration sensitivity of each crystal has a unique magnitude and direction, making any open loop techniques impractical.
Attempts to attenuate the effects of frequency modulation on crystal oscillators is the subject of U.S. Pat. No. 4,555,678, by Galani. However, Galani was not concerned with shock and vibration. The objective of the Galani patent was to degenerate the phase noise of the oscillator and produce a clean low-noise spectrum. A frequency modulated (FM) canceler loop was employed to cancel the phase noise of the oscillator. The FM canceler loop used the oscillator's crystal as a resonator. This approach would not be effective in a vibration environment, since the FM discriminator would exhibit the same response to vibration as the crystal.
Accordingly, there is a current need for an innovative approach to reduce shock and vibration effects in crystal oscillators. Ideally, this innovative approach should present a low cost solution which can be implemented using minimal space and without degrading the performance of the overall system.